Water use is essential in all dairies. Drinking water is indispensable to sustain life of cattle; some additional water is necessary for cleaning and sanitation procedures; moderate further amounts are important in periods of heat stress for evaporative cooling of cows to improve animal production and health; and additional amounts can be used in laborsaving methods to move manure and clean barns by flushing in properly designed facilities. The more water a farm uses, however, the greater the potential for surface runoff and penetration to the ground water, with possible environmental impacts offsite. Heightened environmental concerns and the need for resource conservation have resulted in the implementation of water use permits and other possible regulatory actions. Thus, it is important to ascertain how much water is necessary for all the various procedures to operate a dairy and to look for ways to reuse some water when to do so is feasible.
To give a very rough scale of manure production, typical dairy animals produce about 1.61 ft3 (12.0 gallons) of fresh manure (feces and urine) per 1,000 lb. according to their average live weight per day. Additional solids add to that volume and come from wasted feed, horn, tail, and hoof fragments, free stall bedding, and soil tracked in from outside lots. Additional water is often added to that present in the dairy from water wastage, milking center waste water, water used to clean floors, and “flushing” of alleys. When facilities are designed with concrete floors they are generally engineered with enough of a slope to permit the use of water propelled by gravity to move manure, known as flushing. Flushing is a clean and laborsaving method for moving manure out of the scope of the dairy animal's activities. A flush flume system uses the additional movement of fast flowing water in gutters to transfer manure across the width of a barn or across several barns. A high volume pump creates an adequate flow rate to prevent settling of manure solids and bedding material within the gutters.
The amount of water used for flushing per cow will vary widely, depending on the size and design of facilities and the frequency of flushing, and because of differences in design, the necessary amount of water will need to be individually calculated for each dairy. Generally, usually a “flush” of about 10,000 gallons is needed to clean an alley width of 10 to 16 feet. If four alleys are common for every 400 cows, and the alleys are flushed twice daily, this would be an average use of 200 gal./cow/day. Many dairies use more than two flushings per day. But water is not free, nor is the disposal of waste water. Understanding these volumes, one can readily appreciate the value of any water reclamation, by separating water from the manure it entrains in the gutters, as having a great and positive effect on the economics of a dairy.
Dairy manure is a complex mixture of two vastly different materials-water and undigested feed. Typical material proportions, for instance, are such that in 115 lb. of manure at 15% solid content and a bulk density of 62 lb./ft3 might be mixed with 55 lb. of sand at 95% solid content, and a bulk density of 110 lb./ft3. When added in these proportions, the result is 170 lb. of “heavy” manure at approximately 40% solid content and a bulk density of about 72 lb./ft3. At these proportions, sand held in suspension will not settle out of the manure but can be held largely indefinitely, causing the manure to be highly abrasive when pumped. For that reason, the more dilute the manure, the more suitable for use in the flush or flume dairy.
Gravity enables the first form of reclamation; the managing dairyman simply stores manure-laden water flowing from the gutters in a reception pit (a smaller tank for short-term storage to facilitate separation) or in lagoons (larger ponds that operate identically) and allows, over time, the solids to settle out of the water. Sooner or later, however, reception pits and lagoons become choked with the accumulation of solids. The diversity of waste presents its own problems because the gutters contain not merely manure but other solids such as bedding and other unwanted material draw entrained to the lagoon. Wooden hoof blocks, breeding gloves, plastic pieces, rocks and neck straps will clog pumps and create problems in different points in the system. Yet, because organic waste in the manure had value as fertilizer or as a precursor to fuel, the prudent dairyman would want to recover both of the water and the organic solids separate from inorganic waste. To do so makes pure economic sense.
Generally, the settling rate of particles within manure is affected by the particle's attributes (specifically, the size, shape, and density) relative to the density and viscosity of the liquid being passed through. If a particle is sufficiently large and dense, it will settle out of suspension as a discrete particle, such as a rock drops through a column of water. As a particle settles, it accelerates until the frictional drag on its surface equals the weight of the particle in the suspending fluid. Once friction and gravity are equal, the particle travels downward at a constant velocity, called its terminal velocity.
If there are many particles falling at the same time, the relative velocity between the particle and the fluid passing over it increases, since the space between particles is smaller. If the space is smaller, and the amount of fluid remains the same, the speed at which the fluid flows must increase. The individual particles slow down as the relative velocity of fluid acting against them increases. If the concentration is great enough, the fluid velocity between particles causes the solids to settle as a group with uniform concentration. Settling solids appear as a cloud with a distinct boundary between the top of the cloud and the clear liquid above it. The settling of particles as a cloud is called hindered or zone settling. The speed at which the border between clear liquid and the cloud passes through a column is the settling velocity of the slurry.
Merely separating water from the solids is not adequate; the solids in manure are a nonhomogeneous amalgam which often includes sand as well as an admixture of organic matter and inert solids in diverse sizes. When such solids are suspended in manure, the suspension presents a necessity of sorting the solids in the course of removal. Vegetable matter is compostable, digestible, and flammable. Plastic gloves, metallic debris, and the like, diminish the utility of this vegetable matter. Naturally, it is easiest to remove these items early on and most advantageously, without requiring manual picking from the manure. Because they are so much larger in size than the remaining suspensions within the manure, screens prove advantageous for a first removal mechanism, and are often situated in the gutters themselves or at the gutter outlet to immediately remove them from the flow before any of the remaining smaller particulate matter is addressed.
One inorganic solid that can generally be removed from suitably dilute manure is sand. Removing sand from manure is advantageous in order to preserve rotating machinery operating in the dairy, particularly pumps. Sand is extremely pernicious relative to the equipment in a dairy. In some dairies, sand is used as bedding and, thus in these dairies the manure includes large volumes of sand. Additionally, even when sand is not used for bedding, cows will excrete several pounds per day of grit that are passed through from the feed they receive. Cows also track sand into the dairy. Sand is abrasive, and when moving in manure, sand abrades the surfaces it contacts. A manure handling system as taught herein will include a properly managed sand separation system. In typical embodiments, the gutter system will include either of a sand lane or a settling pit, or grit chamber, to remove heavy sediment from the system before further processing.
Sand separation lanes have come into conventional dairies as effective in the removal of some sand from manure, and by doing so, the preservation of manure handling equipment. Separation is based upon the distinct densities of the matter (i.e. water has a density of 62 lb./ft3 whereas sand densities vary between 120 lb./ft3 to 150 lb./ft3). Where the flow of the heavy manure is slow enough, and where the suspension is suitably diluted with water, sand will settle out of the manure. The settling of sand in separation lanes allows for the periodic removal of sand from the lane in a manner that is more convenient than dredging a lagoon in regular dairy practice. Manure emerging from sand lanes is readily pumpable and it can often be used to motivate manure in gutters, though, as expressed above, most dairymen would want to reclaim the organic matter within the manure.
A typical sand separation lane is about 12 feet wide, several hundred feet long, with a slope of up to ¼% (or 3 inches per 100 feet in length), and an energy dissipater/flow dispersion system at the inlet end. The length of the lane depends on the size of the sand grains; with longer lengths needed for smaller sand grains. Often made of concrete, sand settling lanes are only a few feet in height, and a typical system will have two lanes to allow one to provide for drainage and clean out, while allowing the other lane to be in use. The dairyman can clean out the lane with a front end loader rather than having to clean the same out of storage lagoons.
Organic matter is reclaimed by any of screens, presses, or centrifuges to further remove suspended solids from manure. Used together or as distinct systems, these devices each separate with distinct technologies. For example, a screen is the most common method, often times poised within a gutter to remove the solids, the most common form is positions obliquely to the flow such that debris within the flow exploits its kinetic energy to be strained from the flow and to ride the screen up and out of the flow. Other forms exist as well but the most frequent exploits, in some manner, the oblique screen in order to prevent clogging.
A dewatering screw press also separates liquids from solids. Such a screw press can be used in place of a belt press, centrifuge, or filter. It is a simple, slow moving device that accomplishes dewatering by continuous gravitational drainage. The most commonly known screw press of this design is said to have been invented by famous Greek mathematician Archimedes and is known as the screw conveyor. The screw conveyor consists of a shaft, which is surrounded by a spiral steel plate, similar in design and appearance to a corkscrew. Gravity, too, presses water through perforations in the wall. That water is returned to the dairy while the solids are removed for other use, such as compost.
The most thorough dewatering device, generally used in conjunction with a screen as a preliminary separator is the centrifuge. For purposes of this application, it is useful to refer to two qualities of manure-laden water. The first of these shall be referred to as “light manure,” having approximately one percent organic solids. The second is somewhat less dilute, but often usable for flushing is “heavy manure.” The most common source of heavy manure is the intermediate water discussed above as coming off of the sand lane or other sand separation device. Heavy manure comprises from two to five percent organic solids, or, roughly between two to five times as much of the organic solids present in light manure. Of the two, light manure, carrying a lesser weight of organic solids, is the more desirable for use as flush water. Nonetheless, where light manure is not available, heavy manure, after sand separation where available, can serve as flush water.
Most commonly, a centrifuge is used to separate organic solids from heavy manure to yield light manure for flushing. A centrifuge is a device which employs a high rotational speed to separate components of different densities. The decanter is used for the separation of two or more phases of different specific gravity; in particular for the clarifying of liquids in which suspended solids are present. The separation of solids and liquids takes place within a cylindrical/conical rotating bowl, drawing the more dense organic solids through the heavy manure to accumulate on the periphery for removal from the resulting light manure. Decanter centrifuges have a characteristic that dictates selection of an appropriate size. The spinning mass of the centrifuge is its most notable feature. Great amounts of energy are expended to bring the centrifuge up to its operating rotational speed. Frequent run and stop, deceleration with high inertia load, and overhauling torque tend to expend great amounts of energy and energy has a great cost. The optimally sized decanter is one that can exactly process all of the manure produced in a twenty-four hour period in twenty-four hours. When a centrifuge is idle, it is costly to return it to operation. In operation, the most efficient use of the centrifuge requires it to be consistently and constantly running.
Having described the separation mechanisms that make up a modern dairy manure management system, it is now possible to discuss the problems common with uniting them in a single system-optimization. The duty cycle of one means of separation may not correspond exactly either to use or to conveyance of water or manure within the dairy. Production of manure is, by its nature, intermittent. Most farmers milk their cows twice a day, but in a robotic milking system, cows sometimes choose to be milked four to five times a day. This doesn't necessarily mean that they are producing more milk, nor is it uncomfortable for them; calves would naturally feed at four to six hourly intervals. Flushing, then, must be coordinated with the milking times. Production of manure and collection by flushing varies, then, throughout the day. So, in contrast, by way of example, to the centrifuge, production of manure is neither constant nor consistent over a twenty-four hour interval. To achieve the described efficiency of the centrifuge, there must be some manure held in reserve to feed the centrifuge when the production of manure by the cattle drops off.
To address this fact, tanks are routinely used as buffers for intermediate storage of light or heavy manure within the circuits which the water and courses within the dairy define. The capacity of such tanks, however, is itself problematic. To make each tank large enough to act as a buffer for the needs of the “downstream” processes can be expensive. In a dairy where flushing is not continuous, the tank must be sized to contain a twelve- or twenty-four hour capacity, so as to smooth the fluctuations in flow due to the vagaries of use.
As a construction cost, excavation and erection of tanks is one of the greater expenses in the construction of a modern dairy. Depth is more expensive than breadth in construction. In operation, depth is also expensive as the inevitable dredging of accumulate is more expensive as the depth increases. But volume is a relationship between these two variables. Additionally, the size of the footprint of any tank is an area of land that is not otherwise usable on the dairy. Naturally, the larger the footprint, the greater the capital cost of water reclamation. Economics, being the cruel mistress she is, will dictate that the scarce resources of land ought to be used in the smallest measure, wherever possible.
What the state of the art does not well provide is a means of reducing the size of these tanks between processes for handling water within the dairy. The smaller the tanks necessary, the smaller the capital expenditure and, thus, the more efficient the dairy. The method and system that are described and claimed below provide a more efficient means of water management in dairy husbandry.